Method of etching copper



2,998,557 Patented Oct. 13, 1959 2,908,557 METHOD OF'ETCHING COPPER Otis D. Black, Haddon Heights, N.J., and Leonard H.

Cutler, Philadelphia, Pa., assignors to Radio Corporatlon of America, a corporation of Delaware No Drawing. Application January 7, 1957 Serial No. 632,641

9 Claims. (Cl. 41-42 This invention relates to the etching of copper. More particularly, the invention relates to a method for etching copper wherein the etching solution remains clear and clean, and may be easily regenerated or restored to optimum etching strength. The invention is particularly, but not exclusively, applicable to the etching of copper in printed circuit manufacture.

Usually copper is commercially etched with ferric chloride solutions. It is well known that the etching of copper with ferric chloride has many disadvantages. The etching solution becomes dirty, probably due to the formation of insoluble iron salts, and the etch rapidly changes in its activity. When employed as the etchant in continuous processes, such as the manufacture of a large number of printed circuit boards, sludge or insoluble iron salt formation is rapid. This results in the necessity of shutting down the operation to clean out the equipment. Furthermore, it is not particularly easy to regenerate or maintain the strength of ferric chloride etching solutions. This again means that periodically the operation must be stopped while fresh etchant is provided.

The present invention relates to an improved method of etching copper with acid cupric ion solutions, the etching strength of which may be maintained relatively constant by adding acid to the solution so as to replenish the hydrogen ion concentration. There is no sludge formation and equipment remains clean and unaffected by the cupric ion etch. Likewise, the etching solution itself remains clear and clean until completely spent. An excellent etching rate can be maintained indefinitely and downtime ordinarily required for cleaning equipment is eliminated. Furthermore, copper can be etched over long periods of time at a rate equivalent to that of fresh ferric chloride etching solutions.

Copper is etched by the cupric ion but not the cuprous ion. It has previously been known that this reaction takes place when copper is boiled with, say, cupric chloride solution to furnish the cupric ions. See Fritz Ephraim, Inorganic Chemistry by P. C. L. Thorne and A. M. Ward (Nordeman Publishing Company, Incorporated, New York, 1939). It has now been discovered that the reaction can bemade to take place without requiring boiling.

In accordance with the present invention, using a soluble cupric salt, the cuprous ion formed in the etching reaction may be continuously oxidized back to the cupric state in the presence of an oxidizing agent such as air. Hence the invention makes possible the maintenance of a cupric ion etching solution by oxidizing the cuprous ions being constantly formed by the etching process so as to produce more of the cupric ions needed for etching. The reactions occurring according to the invention are shown by the following equations:

o 2Cu++oxidizing agent=2Cu++ The cupric ion etching solution is preferably acidified with acid having the same negative ion as the cupric salt employed. The general nature of the process is such that, as the concentration of copper ions (both cupric and cuprous) builds up during the etching process, the etching solution may be restored to substantially the initial etching strength and rate by diluting the solution with the acid. Furthermore, this dilution and acidification can be continued indefinitely, the only limitation beingthe increasing volume of the etching solution itself.

It is believed that the mechanism for the overall reactions involved may be explained as follows:

(acid) Cu H O Cu 50, 2H+

The concentrations of the cupric ion and the acid in the solution are primarily determinative of the etching rate. It is preferred to use a cupric ion concentration of about grams per liter but this may be varied between about 45 and grams per liter with less satisfactory results as to etching speed.

The exact acid concentration is not critical but should be sufiicient to provide between about 3 grams and about 5 grams of available hydrogen ion per liter. Hydrochloric acid, for example, when included in one liter of solution in amounts between 100 and 175 grams (calculated as 100% HCl) provides this hydrogen ion concentration. Likewise between 175 and 300 grams per liter of nitric acid or between and 250 grams per liter of sulfuric acid provide the above-mentioned required hydrogen ion concentration. Apparently, the acid is needed not only to provide a common solvent for both the cupric and cuprous ions formed during etching, but also to provide hydrogen ions for the oxidation process (Where the cuprous ions are oxidized to cupric ions). Hence in an etching process Where the etching solution is continuously being regenerated by the oxidation of cuprous ions to form cupric ions there must be at least enough acid present to keep both ions in solution and to provide enough hydrogen ions to provide a favorable medium for the oxidation of the cuprous ions. Too little acid therefore-means that etching may proceed satisfactorily for a short time but then will begin to slow down since the cuprous ions being formed either saturate the solution and begin to precipitate out, or are not oxidized to cupric ions. The upper amount of the acid concentration is limited primarily by such considerations as fuming and the effect on other materials which may be present.

The oxdizing agent may be air, oxygen, chlorine, or hydrogen peroxide to name a few examples. It is only necessary to provide an excess of the oxidizing agent over the quantity required to oxidize the cuprous ions being formed by the etching process, to the cupric state. An excess beyond this minimum is not detrimental. The oxidizing agent may be present as an atmosphere or as a solute in the etching solution. whether a suflicient amount of the oxidizing agent is present or not may be easily determined by checking the cupric ion concentration or by observing the etching rate. If the etching rate begins to slow down, then it is known that not enough cuprous ions are being converted to the cupric state. If the acid concentration is satisfactory, then it is known that a greater amount of oxidizing agent is needed. j

The question of With the passage of time in a continuous etching process the solution becomes more and more concentrated with copper ions (both cupric and cuprous). Hence the etching rate tends to decrease and a precipitate tends to form. As pointed out previously, an excessive cupric concentration results in a slower etching rate while a low acid consentration means that not all of the ions will remain in solution. The acid concentration of course becomes lower due to the oxidation process, one of the products formed being water. It is thus necessary to restore the cupric ion concentration to its predetermined value (i.e., between about 45 and 140 grams per liter) and to re-establish or maintain the proper available hydrogen concentration (i.e., between about 3 and 5 grams per liter of solution). Therefore, the optimum etching solution is maintained by periodically adding acid thereto in an amount slightly in excess of that required by the copper etched. This dilution and acidification may be continued indefinitely, the only limitation, mentioned heretofore, being that of the increasing volume of the etching solution itself and this latter factor may, of course, be taken care of by drawing off some of the etching solution from time to time.

An application of the invention will now be illustrated with the etching of printed circuit boards.

Example I An etching solution was made up consisting of about 46 liters of aqueous solution containing about 200 grams of cupric chloride per liter of solution. An acid concentration of about 150 grams of 100% HCl was established per liter of etching solution by including about 375 grams of 39% concentrated hydrochloric acid per liter. Printed circuit boards were etched with this solution. The printed circuit boards were 18 x 19" and consisted of a copper clad phenolic base. the copper being about .00135" thick. When printed circuits are being manufactured, the printed circuit configuration is printed on the copper foil with an acid resist material. Using the etchant described, these boards were etched as a test in which all of the copper was removed, at a rate averaging less than'three minutes per board. After the complete removal of all the copper from ten such boards the etching solution was restored to its initial strength by the removal of three liters of etching solution and the addition of one and a half liters of concentrated hydrochloric acid and one and a half liters of water. The etching solution was thus maintained at continuous strength for an indefinite run of boards.

The oxygen of the air to which the surface of the etching bath was exposed was sufficient oxidizing agent.

Hydrochloric acid in the concentration used was found to have little or no effect on the phenolic board itself. However, it is necessary to wash the boards very thoroughly after etching to remove all traces of acids. It was found that the scrubbing which is required anvway to remove the photosensitive resist, and soaking in clean running water for one hour is suflicient to remove the acid as well as the resist.

In comparison with etching the same circuit boards by the ferric chloride process. it is found that the cost per board is much less. Furthermore, the down-time required during the ferric chloride process amounted to about four hours for each hundred boards etched as compared with no down-time during the cupric ionacid process.

Example 2 An etching solution was made up consisting of about 100 grams of cupric chloride per liter of water. The acid concentration was established by using about 150 grams of hydrochloric-acid (100% HCl) per liter of the etching solution. Printed circuit boards etched with this solution were identical to those described i t p evious example. The etching time averages less than fourand-a-half minutes per board. It was found that the etching solution was restored to its initial strength after ten boards had been etched by removing three liters of etching solution and adding one-and-a-half liters of concentrated HCl and one-and-a-half liters of water.

Example 3 An etching solution was made up consisting of about 300 grams of cupric chloride per liter of water in which was included about 175 grams of HCl per liter of etching solution. Again the printed circuit boards were identical to those described in connection with Example 1. The boards were etched at a rate averaging less than four inrnutes per board. The etching solution was restored and maintained at its initial strength by removing three liters of etching solution and by adding one-and-a-half liters of HCl and one-and-a-half liters of Water after ten boards had been etched.

When nitric or sulfuric acids are used, instead of hydrochloric acid, a quantity of acid is used sufficient to furnish the same amount of available hydrogen ion as in the case of hydrochloric acid and the corresponding cupric salt, i.e., cupric nitrate or cupric sulfate, is utilized. Since nitric and sulfuric acids are relatively strong oxidizing agents, it is preferable not to use phenolic laminates as the base board when these acids are included in the etching solution. Glass or other more resistant base materials can be used.

To obtain an oxidizing action stronger than that of ordinary atmospheric oxygen, an oxidizing agent may be dissolved in the etching solution. Such an action may be obtained, for example, by adding about 0.5 liter of 30% hydrogen peroxide per 40 liters of solution. This speeds up the etching action somewhat by increasing the speed of formation of cupric ions.

There thus has been shown and described and improved etching method for copper. The solution and etching process described herein are characterized by their extreme cleanness and their ability to be constantly regenerated or restored to an initial predetermined etching strength and rate without requiring a shut down.

What is claimed is:

1. The method of etching copper comprising the steps of: contacting the copper to be etched in the presence of an oxidizing agent with an aqueous solution of a soluble salt capable of furnishing cupric ions in a concentration of from about 45 to grams per liter and having an available hydrogen ion concentration of from about 3 grams to about 5 grams per liter, cuprous ions being formed in said solution as etching progresses, and regenerating said etching solution as needed by maintaining the concentration of said cupric ions and said hydrogen ions within said ranges while etching said copper by adding to said solution water and a substance which will form hydrogen ions.

2. The method of etching copper comprising the steps of: contacting the copper to be etched, in the presence of an oxidizing agent, with an aqueous solution of cupric chloride in a concentration of about 200 grams per liter and having an available hydrogen ion concentration from about 3 grams to about 5 grams per liter, and regenerating said sotlution as needed by maintaining the concentration of said cupric chloride and said hydrogen ions within and at said values while etching said copper by adding to said solution water and a substance which will form hydrogen ions.

3. The method of etching copper comprising the steps of: contacting the copper to be etched in the presence of an oxidizing agent with an aqueous solution of cupric chloride and hydrochloric acid, and regenerating said etching solution as needed by maintaining predetermined concentrations of'said cupric chloride and said acid while etching said copper.

4. The method according to claim 3 wherein said con- 5 eentrations are maintained by adding water and hydrochloric acid to said solution while etching said copper.

5. The method of etching copper comprising the steps of: contacting the copper to be etched in the presence of an oxidizing agent with an aqueous solution of cupric chloride in a concentration of between 100 to 300 grams per liter and hydrochloric acid in a concentration of between 100 to 175 grams per liter, calculated as 100% HCl, cuprous ions being formed in said solution as etching progresses, and regenerating said solution as needed by maintaining said concentrations of said cupric chloride and said acid in said solution by adding water and hydrochloric acid thereto while etching said copper.

6. The method according to claim 5 wherein the concentration of said cupric chloride in said solution is about 200 grams per liter.

7. The method according to claim Swherein the concentration of said cupric chloride in said solution is about 200 grams per liter and the concentration of said hydrochloric acid is about 150 grams per liter.

8. The method of removing copper from a base member comprising the steps of: preparing an etching solution consisting essentially of from 100 to 300 grams of cupric chloride and an available hydrogen ion concentration of from about 3 to about 5 grams per liter of water, 25

immersing said base member into said solution in the presence of an oxidizing agent until said copper is removed therefrom, and constantly regenerating said solution as needed by adding more water and a substance 6 which forms hydrogen ions to said solution as needed to maintain the concentration of said cupric chloride and said hydrogen ions in said solution.

9. The method of removing'copper from a base member comprising the steps of: preparing an etching solution consisting essentially of from 100 to 300 grams of cupric chloride and from 100 to 175 grams of hydrochloric acid, calculated as 100% HCl, per liter of water,

immersing said base members into said solution in the presence of an oxidizing agent until said copper is removed therefrom, and constantly regenerating said solution as needed by adding more water and hydrochloric acid to said solution as needed to maintain the concentration of said cupric chloride and said acid in said solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,233,546 Meulendyke Mar. 4, 1941 2,291,202 Bassett et a1. July 28, 1942 2,378,052 Waldman et al June 12, 1945 FOREIGN PATENTS 484,159 Great Britain Apr. 28, 1938 OTHER REFERENCES Formulas, Processes and Trade Secrets; pub. by Norman W. Henley Publishing Co., New York, 1948, page 323, column 2, par. 3. 

8. THE METHOD OF REMOVING COPPER FROM A BASE MEMBER COMPRISING THE STEPS OF: PREPARING AN ETCHING SOLUTION CONSISTING ESSENTIALLY OF FROM 100 TO 300 GRAMS OF CUPRIC CHLORIDE AND AN AVAILABLE HYDROGEN ION CONCENTRATION OF FROM ABOUT 3 TO ABOUT 5 GRAMS PER LITER OF WATER, IMMERSING SAID BASE MEMBER INTO SAID SOLUTION IN THE PRESENCE OF AN OXIDZING AGENT UNTIL SAID COPPER IS REMOVED THEREFROM, AND CONSTANTLY REGENERATING SAID SOLUTION AS NEEDED BY ADDING MORE WATAER AND A SUBSTANCE WHICH FORMS HYDROGEN IONS TO SAID SOLUTION AS NEEDED TO MAINTAIN THE CONCENTRATION OF SAID CUPRIC CHLORIDE AND SAID HYDROGEN IONS IS SAID SOLUTION. 